1. Field of the Invention
The present invention relates in general to a differential-pressure casting method, and more particularly to a technique for regulating the temperature of a molten metal in a series of casting operations with loading and unloading of casting molds.
2. Discussion of the Prior Art
As one method of casting, there is known a differential-pressure casting process in which a cavity in a mold is filled with a molten metal, through a pouring tube one end of which is connected to a sprue hole of the mold and the other end of which is immersed in the mass of molten metal accommodated in a suitable container or ladle. The molten metal is poured into the mold cavity through the pouring tube, due to a difference between the pressures to which the mold and the molten metal in the container are exposed. With the mold cavity filled with the metal, an article made of the metal has a shape corresponding to that of the mold cavity. For example, such differential-pressure casting process is disclosed in JP-A-57-118853, wherein a pressure-tight container accommodating a mass of molten metal so as to maintain it at an elevated temperature is pressurized by application of up to one atmospheric pressure, to feed the molten metal into the mold cavity through the pouring tube, due to the elevated pressure in the molten metal container. In another known differential-pressure casting process, the mold placed in a pressure-tight vessel is exposed to a reduced pressure, so that the molten metal is introduced into the mold cavity through the pouring tube, under the action of the reduced pressure in the evacuated pressure-tight vessel.
Such differential-pressure casting process using the pouring tube whose lower end portion is immersed in the mass of molten metal to fill the mold cavity is free from otherwise possible introduction into the mold cavity of air, and slag and flux floating on the surface of the molten metal, during the filling of the mold cavity. Further, the differential-pressure casting process permits the molten metal to be continuously fed into the mold cavity even after the solidification of the already introduced metal in the cavity is started, whereby the cast article produced is effectively protected from defects such as blowholes which arise from the metal shrinkage upon solidification. The differential-pressure casting process also permits directional solidification of the molten metal, leading to a sound casting with high air-tightness. Moreover, the process is capable of regulating the elevated pressure in the molten metal container or the reduced pressure in the mold vessel, and therefore permits the molten metal to be sufficiently slowly introduced into the mold cavity, so that sand cores may be used in the mold cavity, as needed, when the article to be cast has a complicated shape. These are some of various advantages of the differential-pressure casting process.
In a casting process as described above, the cast article may suffer from defects such as blowholes or pinholes, particularly when the temperature of the molten metal is excessively high, and may also suffer from "misrun" when the temperature is too low. The "misrun" means incompletely formed casting due to premature solidification of the molten metal before the mold is filled. Thus, the adequate regulation of the molten metal temperature is important to assure consistently high quality of the castings. To this end, the casting is usually performed while measuring and regulating the molten metal temperature. The temperature of the molten metal is generally measured or detected by using a thermocouple made of two dissimilar conductors such as a combination of platinum and platinum/rhodium alloy, or a combination of alumel (Ni/Al/Si/Mn alloy) and chromel (Ni/Cr alloy). To measure the molten metal temperature, the detecting portion of the thermocouple is immersed in a mass of the molten metal.
Where a thermocouple as mentioned above is used to measure the temperature of the molten metal, however, the detecting portion of the thermocouple immersed in the molten metal mass is subject to oxidation, corrosion or contamination due to the molten metal and slag, and tends to suffer from severe physical or chemical deterioration, leading to extremely shortened life expectancy of the thermocouple. The shortening of the service life of the thermocouple is serious particularly when the temperature of the molten metal is comparatively high, as in the case of casing of iron or nickel alloy materials. Consequently, when a series of casting operations is continuously or intermittently performed with replacement of casting molds, accurate measurement and adequate regulation of the molten metal temperature by the thermocouple are possible for only the first casting cycle or shot, and are difficult for the subsequent casting cycles or shots. Accordingly, the quality of the casting is lowered with an increase in the number of the continuous casting cycles performed.